MNT: Fix test for NumPy 1.19
MNT: Fix test for NumPy 1.19

We were using pytest to catch *and check* a warning coming from numpy,
which we no longer get. Just use a simple warning filter since it's not
part of *our* API anyway.

dopplershift/MetPy,ShawnMurd/MetPy,Unidata/MetPy,Unidata/MetPy,dopplershift/MetPy